1. Field of the Invention
The present invention relates to a variable capacitor, a variable inductor, and a radio frequency circuit module, and additionally relates to a variable capacitor and a variable inductor suitable for use as passive elements in a radio frequency circuit module, and to radio frequency circuit module having this variable capacitor and variable inductor.
2. Related Art
Radio frequency circuits used in radio communications devices such as portable telephones are formed by a large number of components, such as semiconductor transistors, IC chips, and passive circuit, elements such as resistors, capacitors, and inductors. In order to achieve compactness in such apparatuses, modularization techniques, which integrate those components amenable to such integration on one substrate or within one package, is widely used. For example, in an RF amplifier module used in a portable telephone, a transistor which amplifies a high-frequency (radio frequency, RF) signal, an input impedance matching circuit for the purpose of supplying an RF signal to the transistor efficiently, and an output impedance matching circuit for the purpose of extracting the amplified RF signal with good efficiency are formed on one substrate. A multilayer board made of glass-ceramic or resin is often used as the board for this circuitry.
In this type of RF circuit module, in the case in which the impedance of a matching circuit includes a deviation from the designed value because of variations in the process of fabricating the board or transistor, it is generally necessary to adjust the impedance after fabrication. Given this situation, technology has been proposed in the past to enable adjustment of the impedance.
For example, in Japanese unexamined patent publication (KOKAI) No. 11-176987, there is disclosure of technology which provides a plurality of electrode pads on the surface of a multilayer board, impedance-adjusting capacitors being provided between a ground electrode layer inside the multilayer board and the electrode pads. In this conventional art, a laser or the like is used to selectively remove or cut an electrode pad, thereby causing a change in the surface area of the electrode pad. An alternate method is that of cutting the connection part of a desired electrode pad, thereby selecting a capacitor to be connected to the transmission line of the matching circuit. By using such a method, the capacitance that is connected to the transmission line of the matching circuit is caused to change, thereby enabling adjustment of the impedance of the matching circuit.
In recent years, there is an increasing demand for so-called multiband and multimode radio communications apparatuses, which can accommodate communication on a plurality of frequency bands and using a plurality of modes. For example, in the digital cellular system used in Europe, both the 900-MHz GSM (Global System for Mobile Communication) and the 1.8-GHz DCS (Digital Cellular System) are widely used, making it necessary to have a portable terminal capable of using both modes. In an RF circuit module used in such a multiband or multimode radio communications apparatus, it is necessary to achieve impedance matching for each frequency band and operating mode.
Given the above, as reported in the 1999 IEEE MTT-S International Microwave Symposium Digest, pp. 1397-1400 by Yamamoto et al, two sets of amplifiers and matching circuits are separately provided to accommodate both GSM and DCS, these being selectively used to achieve impedance matching.
In the prior art disclosed in the Japanese unexamined patent publication (KOKAI) No. 11-176987, both the electrode pads and transmission line of the matching circuit are formed on the surface of a multilayer board. For this reason, there is an increase in the mounting surface area of the matching circuit, making this approach unsuitable for use in compact RF circuit modules.
Additionally, in this prior art, when adjusting the impedance a laser or the like is used to remove or cut an electrode pad, or the connection part of an electrode pad is cut. For this reason, the change in the impedance of the matching circuit is irreversible, making it impossible to use this approach to achieve impedance matching in multiband or multimode applications.
The Japanese unexamined Utility Model publication (KOKAI) No. 64-16669 discloses a multilayer circuit board which provides a plurality of capacitors. In this prior art, when changing an overall capacitance of the capacitor, the wiring formed on the multilayer circuit board is changed. Therefore, it is impossible to obtain reversible impedance change in this circuit.
In technology for providing separate circuitry for each frequency band or each operating mode, although use is possible in a multiband or multimode radio communications apparatus, there is an increase in the number of components used in the RF circuit module, thereby causing the problem of difficulty in achieving a compact, low-cost RF circuit module.
Stated differently, in the conventional art, it is not possible to find and use a reactance element that not only is amenable to reversible impedance change, but also that is compact.
Accordingly, it is an object of the present invention is to provide a variable capacitor capable of reversible change in the capacitance, and additionally, capable of an reversible change in impedance, and also capable of being implemented in a compact form.
Another object of the present invention is to provide a variable inductor capable of reversible change in the inductance, and by extension, capable of an reversible change in impedance, and also capable of being implemented in a compact form.
Yet another object of the present invention is to provide an RF circuit module capable of easy adjustment for a deviation in impedance caused by variations in manufacturing, capable of accommodating multiband and multimode operation, and capable of compact, low-cost implementation.
In order to achieve the above-noted object, the present invention adopts the following basic technical constitution.
Specially, the first aspect of the present invention is a variable capacitor comprising: a multilayer circuit board having a plurality of dielectric layers; a first conductive plate, provided within the multilayer circuit board, for serving as one electrode of the variable capacitor; a second conductive plate, provided within the multilayer circuit board, for serving as the other electrode of the variable capacitor; a plurality of third conductive plates provided between the first conductive plate and the second conductive plate; and a plurality of switching means provided for grounding the third conductive plates selectively.
The second aspect of the present invention is a variable capacitor comprising: a multilayer circuit board having a plurality of dielectric layers; a first conductive plate, provided within the multilayer circuit-board, for serving as one electrode of the variable capacitor; a second conductive plate, provided within the multilayer circuit board, for serving as the other electrode of the variable capacitor; a plurality of third conductive plates provided between the first conductive plate and the second conductive plate; a plurality of terminals provided on an outer surface of the multilayer circuit.board; a plurality of via holes connecting the a plurality of third conductive plates and the a plurality of terminals, respectively; and a plurality of switching means provided for grounding said third conductive plates via said terminals selectively.
The third aspect of the present invention is a variable inductor comprising: a multilayer circuit board having a plurality of dielectric layers; a first conductive plate, provided within the multilayer circuit board, for serving as a transmission line; a second conductive plate, provided within the multilayer circuit board, for serving as a grand plate of the transmission line; a third conductive plate, provided between the first conductive plate and second conductive plate, for serving as a grand plate of the transmission line; and a plurality of switching means provided for grounding either one of the second and the third conductive plate selectively.
The fourth aspect of the present invention is a variable inductor comprising: a multilayer circuit board having a plurality of dielectric layers; a first conductive plate, provided within the multilayer circuit board, for serving as a transmission line; a second conductive plate, provided within the multilayer circuit board, for serving as a grand plate of the transmission line; a third conductive plate, provided between the first conductive plate and second conductive plate, for serving as a grand plate of the transmission line; a terminal provided on an outer surface of the multilayer circuit board; and a via hole connecting the third conductive plate and the terminal.